Numerical and experimental thermofluid investigation of different disc-type power transformer winding arrangements

In this paper, measurements are carried out on four different washer arrangements of an ON disc-type power transformer winding scale model. The experimental setup comprises a closed cooling loop with all the main components generally found on a power transformer and it is equipped with both thermal and flow sensors. Moreover, 3D Conjugate Heat Transfer simulations of the entire cooling circuit are performed using a commercial CFD solver and the computed oil flow rates and winding temperatures are compared with the experimental data for both uniform and non-uniform heat loss distributions. The experimental results show that the reduction of the number of washers in the tested scale model winding increases the total oil flow rate but this effect is overridden by a higher flow maldistribution in the radial ducts of a pass. Thus, the discs temperatures increase with the removal of washers and this effect is particularly marked for a non-guided winding arrangement where an almost stagnant flow is observed in several radial cooling ducts. The CFD results show the same trend but the numerical model consistently underpredicts the total oil flow rate circulating in the closed cooling circuit. This underestimation by the CFD model causes, for certain winding arrangements, significant errors in the evaluation of the average and hot-spot temperatures. For this reason, numerical simulations with a reduced computational domain (i.e., winding region only) are also performed by specifying the measured oil flow rate and temperature as inlet boundary conditions. In this case, the accuracy of the numerical model is significantly improved as the predicted average and hot-spot winding temperatures are within 3 degrees C of the corresponding measured values. This result is reassuring since the majority of published numerical thermofuid studies on transformer windings are performed on the windings region only and boundary conditions are specified at the inlet, thus avoiding the simulation of the entire cooling loop.